Usher syndrome IIIA (USH3A) is an autosomal recessive disorder characterized by progressive loss of hearing and vision due to mutation in the clarin-1 (CLRN1) gene. One of the most common genetic alterations in USH3A patients is the Y176X mutation in CLRN1, a presumptive null mutation. Lack of an animal model has hindered our ability to understand the function of the protein (CLRN1) and the pathophysiology associated with USH3A. We generated a Clrn1 knockout (Clrn1-/-) mutation in the mouse, and our results show that the inner ear phenotype in the knockout animals is similar to the clinical presentation seen in patients harboring the Y176X mutation. The Clrn1-/- mouse should, therefore, be a good model for ear disease in USH3A patients and, to our knowledge, it is the first such model to become available. Preliminary work shows that Clrn1 mRNA was expressed as early as embryonic day 16.5 in the auditory and vestibular hair cells and associated ganglionic neurons. At 2-3 weeks postnatal (P14-21), Clrn1-/- mice showed elevated auditory brainstem response (ABR) thresholds and prolonged peak and interpeak latencies. By P21 ~70% of Clrn1-/- mice had no detectable ABR, and by P30, almost all Clrn1-/- mice were deaf. Distortion product otoacoustic emissions were not recordable from Clrn1-/- mice. Vestibular function in Clrn1-/- mice mirrored the cochlear phenotype, although it deteriorated more gradually than cochlear function. Disorganization of OHC stereocilia was seen as early as P2, and by P21, OHC loss was observed. Also, previous studies have shown that USH3A patients with a single mutant allele of USH1B (the myosin 7A gene) exhibit the USH1 phenotype, suggesting a genetic interaction between CLRN1 and MYO7A. In sum, we hypothesize that Clrn1 is necessary for hair cell function and hair cell to afferent communication, and that CLRN1 interacts with MYO7A to mediate its function. We propose 4 specific aims to test those hypotheses. The purposes of these aims are: (1) To dissect the cell type-autonomous functions of Clrn1, (2) To assess the development of ribbon synapse in Clrn1-/-hair cells, (3) To determine whether CLRN1 is expressed in the bundle and synaptic region of hair cells, and (4) To demonstrate biological interaction between CLRN1 and MYO7A. The proposed work will define the function of CLRN1 in the inner ear, shed light on its biological interaction with other proteins, help us better understand the pathophysiology linked to deafness in USH3A patients, and provide a rationale to develop therapeutic strategies to delay or abolish the progression of hearing impairment in the mouse model of USH3A.